The act of inserting some sort of baton i.e. drumstick, chopstick, dildo etc. etc. up your arse, leaving a good length sticking out. Your opponent does the same and you hit each others baton with your own to make you opponents fall out. The first one to lose his/her baton is the loser, allowing the winner to run his baton under the loser's nose getting a good whiff.

Quote Dirty sanchez 'You put a jellyfish on my T-Shirt, cracked an egg on my face and wrote BumFight(s) on my forehead'

Also 'I challenge you to a bum fight - a drumstick shall be my weapon of choice'

Easy to start: 1.) Find a minimum of at least 2 Bums.
2.)Go tell Bum #1 that Bum #2 was the guy responsible for putting him on the street and that he stole the last little bit of Thunderbird, and his last cigarette filter, you know, the one that has 3 to 4 grains of tobacco on it.
3.)Sit back and watch the crust chip away as they come out swingin'.
Sometimes it'll work when you walk through Grand Central Station and drop a dollar in change in front of a gaggle of Bum.

I Got filthy when I started a couple of Bum Fights and their crust was flying!